Unstable/stable release model

Unstable 2.3.x releases will be cut fairly frequently directly from Jackrabbit trunk, and a stable 2.4 release branch will be created later on. At current rate it looks like the 2.4 branch can be created before the end of 2011.

Once the 2.4 branch has been created, trunk will switch to 2.5.x and continue progressing towards 2.6, etc.